The Most Mysterious Star in the Galaxy

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It has been dubbed the most mysterious star in the galaxy. The star, more
than 1,200 light-years away in the constellation Cygnus the Swan, flickers and
dims in a way never seen before.

The starlight dips in an irregular pattern, suggesting that something is
intermittently blocking the star. This bizarre behaviour, first reported in
autumn 2015 and not seen in any other star, has scientists baffled. Researchers
have proposed a myriad of explanations, including black holes, comet swarms,
and interstellar clouds. But a conclusive answer remains elusive. And one
hypothesis in particular has raised eyebrows: aliens.

Yes, aliens.

Perhaps, researchers have posited, an advanced alien civilisation has
constructed a vast structure encircling their star, maybe an enormous power
plant that harnesses the star's energy. When parts of the structure pass in
front of the star, it creates dips in the starlight.

If true, such an alien megastructure would be the biggest discovery in
history. And if you were to believe some of the headlines over the past year,
you might think proof was imminent. Of course, much was overhyped, and to be
clear, no scientists are claiming that this star, named KIC 8462852, is evidence
of ET.

"Even people who were maybe annoyed at the alien angle have to admit
that it's a really good mystery," says Jason Wright, an astronomer
at Penn State University in the US.

KIC 8462852's brightness dropped by as much as 22 percent (Credit:
Nasa/JPL-Caltech)

This is not the first time astronomers have evoked aliens to explain
something weird in space. When astronomer Jocelyn Bell Burnell discovered rapid
pulses of radio waves in 1968, she labeled the mysterious signals
"LGM", for "little green men", to indicate that they might
be extraterrestrial messages. But she was being tongue-in-cheek, and
astronomers quickly realised these signals were coming from rapidly-spinning
neutron stars, which they would eventually call pulsars.

For four years, Kepler stared at one patch of sky roughly the size of
your extended fist

But unlike many of the previous false alarms, which usually assumed an
alien communication beacon, the megastructure hypothesis is one with relatively
specific predictions. In his 1937 science fiction novel Star Maker,
Olaf Stapledon speculated that an advanced civilisation, thirsty for energy,
would eventually need to extract power from its star. To do so, it would have
to build a structure that encloses its star. Inspired by this idea, the
physicist Freeman Dyson proposed in 1960 that the search for intelligent life
should target these structures, which are now often referred to as Dyson
Spheres.

As recently as 2005, an astronomer named Luc Arnold suggested that
such a structure could also be used to create a flickering signal that
announces the existence of an alien civilisation, like an interstellar Morse
code.

To detect the subtle dips in starlight caused by one of these structures,
you would need a sensitive telescope trained on stars for a long time. As it
turns out, this telescope already exists.

The Kepler Space Telescope, launched in 2009, was designed to find
planets around other stars. For four years, Kepler stared at one patch of sky
roughly the size of your extended fist, watching for dips in starlight caused
by planets passing in front of their stars. Kepler has been a triumph,
discovering more than 2,300 of the 3,400 confirmed planets known so far.

But Wright realised the telescope could also detect "alien
megastructures", if they exist. He was pondering that possibility when he
learned about a particularly puzzling star in the Kepler database: KIC 8462852.

They called the star the Where's The Flux Star, or – since astronomers
love clever acronyms – the WTF Star

For years, astronomer Tabetha
Boyajian, now at Louisiana State University, and her colleagues had
been grappling with this star. In fact it is now called Boyajian's Star, or
sometimes Tabby's Star. A group of citizen scientists, through a program
called Planet Hunters,
discovered this star's peculiar, irregular dips and brought it to her
attention. In one of the dips, the star's brightness plunged by 22%: a huge
drop that suggests something expansive was blocking it. For example, the planet
Jupiter is the largest in our Solar System, but such a planet would cause only
a 0.5% drop in brightness.

No one could figure out what was going on. "For a scientist, that's
exciting because it's new," Boyajian says. "But it's also
frustrating."

When her team published
their data in 2015, they called the star the Where's The Flux
Star, or – since astronomers love clever acronyms – the WTF Star. They
tentatively suggested a swarm of comets might be creating the dips.

But when Boyajian showed Wright the data, he realised that the dips were
consistent with an alien
megastructure. That is, if an alien megastructure exists, it might make
dips similar to those seen from Boyajian's Star.

Several thousand known exoplanets have now been discovered (Credit:
Nasa/Jpl-Caltech)

The dips themselves are not that weird. Kepler and other telescopes have
seen many stars with sudden and brief drops in brightness. For instance, young
stars are often surrounded by a disk of gas and dust. These disks are the raw
material from which planets can form, and they obstruct light. But the strange
thing about Boyajian's Star is that it is a run-of-the-mill, middle-aged star.

It also does not emit any infrared radiation. Astronomers expect anything
that could block starlight, such as a disk of dust, would also be close to the
star, so it would heat up and glow in infrared. But astronomers have not
detected any infrared radiation, suggesting there is no disk at all.

Maybe the star is devouring a planet

Then in January 2016, the star got even weirder. Bradley
Schaefer, an astronomer at Louisiana State University, found that in
addition to the short-term dips, the star appeared to have dimmed 15% over the
last century. Schaefer had gone through the archives, looking at images of the
star taken on photographic plates dating back to 1890. But because the analysis
relies on rare expertise in photographic plates, not all astronomers were
convinced.

More convincing evidence would come in a few months. Benjamin Montet at
the University of Chicago and Josh Simon at
the Carnegie Institution for Science looked through Kepler's four-year trove of
data, and found that Boyajian's Star dimmed by 3% over four years. That does
not confirm Schaefer's longer-term analysis, but the results are consistent.

"This seems to lend a lot of support to what Schaefer claims – the
fact that we observe dimming similar in nature to what he observes,"
Montet says.

Young stars are often surrounded by gas and dust (Credit:
Nasa/Esa/Nota/STScI/Esa)

This slow dimming, punctuated with dips, makes the star even more
difficult to explain. For example, it is hard to see how a swarm of comets,
Boyajian's initial suggestion, could account for long-term dimming.

The megastructure hypothesis can potentially explain the dimming. Wright
offers one possible scenario: if the structures consist of a multitude of
objects such as solar panels, which orbit the star at different distances and
speeds, they could conceivably dim the star over years or decades, like a
slow-moving flock of starlings obscuring the sky.

This debris could be causing the dips

But recently, taking into account this gradual dimming, a new proposal
has gained favour among some astronomers. Maybe the star is devouring a planet.
In this scenario, a planet was once orbiting Boyajian's Star. Another, nearby
star exerted a gravitational tug on the planet, altering its orbit enough to
send it plunging into Boyajian's Star. As the planet approached the star, its
outer layers – or, perhaps, its moons – got stripped away, leaving behind a
trail of debris that is still in orbit around Boyajian's Star. This debris
could be causing the dips.

When the planet finally crashed into the star, temperatures rose and the
star brightened. After a while, as the energy dissipated, the star began to
fade back to its normal brightness. It is this gradual dimming that Montet and
Simon, and perhaps Schaefer, now detect.

The planet-crashing idea is promising because it explains both the
long-term dimming and the short-term dips. "Our scenario is really one of
the first ones to connect them into a common cause," says Brian Metzger, an
astrophysicist at Columbia University who helped conceive the new hypothesis.

There are millions of stars like Boyajian's

But it also suggests these planetary collisions are more common than
anyone would have thought. Boyajian's Star may be one-of-a-kind among Kepler's
nearly 200,000 stars. But our galaxy has about 100 billion stars, so if you
extrapolate the numbers, there should be millions of stars like Boyajian's, and
at least thousands of planetary collisions should happen every year.

Because astronomers had only noticed the dips after the fact, no one has
ever seen them happening in real-time. To catch these dips in action, Boyajian
is leading an effort to use the Las Cumbres Observatory, a global network of
telescopes, to monitor the star. She is also enlisting the help of amateur
astronomers through the Association of American Variable Star Observers.

Once they detect a dip, astronomers can point other telescopes to study
the star in greater detail. For example, the Spitzer Space Telescope could
identify bursts of infrared light, a potentially crucial clue.

According to the planet-colliding hypothesis, debris blocks light when it
passes in front of the star during its long, elliptical orbit. The debris would
also heat up and radiate in the infrared. But the radiation is temporary,
stopping once the debris' orbit takes it further from the star. Capturing this
infrared flash would lend support to a planet collision.

Gaia is measuring the distances to a billion of the nearest stars
(Credit: Esa/Gaia/DPAC)

Astronomers are also looking forward to the results from the European
Gaia mission, which is measuring the distances to a billion of the nearest
stars. If Gaia can pinpoint the precise distance to Boyajian's Star,
astronomers will be able to understand its true brightness. By comparing that
to the star's observed brightness, researchers can determine whether the
gradual dimming is simply the star returning to normal following a planetary
crash. However, that data may not be available until at least 2019.

The alien megastructure hypothesis is still arguably a scientific one

Future observations will answer more questions. For example, another
plausible theory is that a big cloud of dust and gas is drifting by, somewhere
between Earth and Boyajian's Star. Denser clumps may cause the dips, and a
transition from a thinner to thicker part of the cloud causes the long-term
dimming.

But what about aliens? In principle, they are still on the table. Setting
aside the sensationalised media coverage, the alien megastructure hypothesis is
still arguably a scientific one, and a valuable speculative exercise. At the
very least, Wright says, it gives alien-hunting astronomers a target at which
to point their radio telescopes, in their search for extraterrestrial signals.

Initial observations using the Search for Extra-Terrestrial Intelligence
(SETI) Institute's Allen Telescope Array in California have turned up empty.
However, in January 2017, Wright, Boyajian and Andrew Siemion of the University of California,
Berkeley, used the Green Bank Telescope in the US for a more thorough
search. They are now analysing the data.

Artist's impression of Gaia mapping the stars of the Milky Way

(Credit:
EsaATG medialab; background: Eso/Brunier)

For others, such as Metzger, who proposed the planetary-collision
hypothesis, the prospect of aliens brought their attention, and maybe some
derision, to a star they might otherwise have ignored. "I was a little bit
distraught that the alien megastructure was considered a plausible
explanation," he says. "If we could find some other mechanism, that's
vastly preferable."

Solving this mystery will probably take a few more years

Most scientists, including Wright, would likely agree that alien
megastructures are a last-resort explanation. But even if the answer is
non-alien, scientists say, it will still reveal unexpected and important
insights into the Universe: whether it is about planetary collisions, globs of
gas in the interstellar medium, or something else altogether.

"The fact that we've gone this long and can't come up with a
satisfactory explanation is interesting," says Eric Mamajek, an
astronomer at NASA's Jet Propulsion Laboratory. "We're going to learn from
it, no matter what it is."

Solving this mystery will probably take a few more years, Mamajek says.
The answer could turn out to be singular and unique, or it could be something
more fundamental. In the near future, more powerful instruments will monitor
the sky with unprecedented sensitivity, potentially discovering more objects
like it.